Symmetric Lorentzian and asymmetric Fano line shapes are fundamental spectroscopic signatures that quantify the structural and dynamical properties of nuclei, atoms, molecules, and solids. This study introduces a universal temporal-phase formalism, mapping the Fano asymmetry parameter q to a phase φ of the time-dependent dipole response function. The formalism is confirmed experimentally by laser-transforming Fano absorption lines of autoionizing helium into Lorentzian lines after attosecond-pulsed excitation. We also demonstrate the inverse, the transformation of a naturally Lorentzian line into a Fano profile. A further application of this formalism uses quantum-phase control to amplify extreme-ultraviolet light resonantly interacting with He atoms. The quantum phase of excited states and its response to interactions can thus be extracted from line-shape analysis, with applications in many branches of spectroscopy.

Esta es la versión de autor del artículo publicado en: This is an author produced version of a paper published in: The concerted motion of two or more bound electrons governs atomic 1 and molecular 2,3 non-equilibrium processes and chemical reactions. It is thus a long-standing scientific dream to measure and control the dynamics of two bound and correlated electrons in the quantum regime. At least two active electrons and a nucleus are required to address such quantum three-body problem 4 for which analytical solutions do not exist, a condition that is met in the helium atom. While attosecond dynamics were previously observed for singleactive electron/hole cases 5-7 , such time-resolved observation of two-electron motion thus far remained an unaccomplished challenge. Here, we measure a 1.2-femtosecond quantum beat among low-lying doubly-excited states in helium and use it to reconstruct a correlated two-electron wave packet. Our experimental method combines attosecond transientabsorption spectroscopy 5,7-9 at unprecedented high spectral resolution (20 meV s.d. near 60 eV) with an intensity-tuneable visible laser field to couple 10-12 the quantum states from the weak-field to the strong-coupling regime. Employing the Fano resonance as a phasesensitive quantum interferometer 13 , we demonstrate the coherent control of two correlated electrons, which form the basis of most covalent molecular bonds in nature. As we show, such multi-dimensional spectroscopy experiments provide benchmark data for testing fundamental few-body quantum-dynamics theory. They also light a route for site-specific measurement and control of metastable electronic transition states that are at the heart of fundamental reactions in chemistry and biology.Electrons are bound to atoms and molecules by the Coulomb force of the nuclei. Moving between atoms, they form the basis of the molecular bond. The same Coulomb force, however, acts repulsively between the electrons. This electron-electron interaction represents a major challenge in the understanding and modelling of atomic and molecular states, their structure and in particular their dynamics 2,3,14 . Here, we focus on the 1 P sp 2,n+ series 15 of doubly-excited states in helium below the N = 2 ionization threshold. They are excited by a single-photon transition from the 1 S 1s 2 ground state by the promotion of both electrons to at least principal quantum number n = 2. The states autoionize due to electron-electron interaction and their spectroscopic signature manifests as asymmetric non-Lorentzian line shapes. The latter were first observed in the 1930s 16 and attributed 17 , by Ugo Fano, to the quantum interference of bound states with the continuum to which they are coupled (Fig. 1c,d). The coupling is described by the configuration interaction V CI with the single-ionization continuum |1s,p, where one electron is in the 1s ground state and the other one is in the continuum with kinetic energy . The magnitude of V CI determines the lifetimes of the transiently bound states. In our case,...

The lifetime of interatomic Coulombic decay (ICD) [L. S. Cederbaum et al., Phys. Rev. Lett. 79, 4778 (1997)] in Ne2 is determined via an extreme ultraviolet pump-probe experiment at the Free-Electron Laser in Hamburg. The pump pulse creates a 2s inner-shell vacancy in one of the two Ne atoms, whereupon the ionized dimer undergoes ICD resulting in a repulsive Ne+(2p(-1))-Ne+(2p(-1)) state, which is probed with a second pulse, removing a further electron. The yield of coincident Ne+-Ne2+ pairs is recorded as a function of the pump-probe delay, allowing us to deduce the ICD lifetime of the Ne2(+)(2s(-1)) state to be (150±50)  fs, in agreement with quantum calculations.

We have calculated the energy distributions of sputtered Nb and Cu atoms ejected from amorphous targets under low-energy Ar bombardment. A formula based on elementary kinetic gas theory is used to calculate the subsequent energy loss of the ejected atoms due to collisions in the sputtering gas. The energy distributions of the sputtered atoms arriving at the substrate is compared with the distributions obtained using thermal evaporation techniques. This comparison indicates that the preparation of epitaxial metallic films, such as Layered Ultrathin Coherent Structures using sputtering techniques may have fundamental advantages over thermal evaporation.